Near Ground Wind Speeds 1991
In most cases, the greater aerodynamically-effective roughness of the environment structures have been ascertained reductions of the wind speed for all urban structure types-groups both at daytime as well as in the night in comparison to the reference station Tempelhof airport.
Map 04.03.1 Near Ground Wind Speeds by Day
The lowest wind speeds are displayed in the small closed courts of the inner city boroughs Kreuzberg, Schöneberg and Prenzlauer Berg. Garden court structures and forests reach of course a better, if only very slight ventilation level with reductions of about 80 % compared to the reference value. In forests the structure of the tree stock prevents the wind’s intrusion in the stem area, rather it is diverted chiefly above the crowns. Thus the emission risk in these areas is high. Inner city green spaces and vacant spaces with sparse bush – and tree vegetation are better ventilated, with a wind speed of up to 30 % of that measured at Tempelhof. However increased emission risk exists here also. This is especially true of street intersections because of the shielding effect of the vegetation. The outer areas with lower development density and height as well as sparser vegetation structures and small business areas display favorable ventilation conditions. Farmland, open vacant areas and green spaces are with up to 80 % of the wind speed measured in Tempelhof very well ventilated.
The highest wind speeds for the built-up areas were measured in high-rise settlements. Turbulences, jet effects and wind canalization are produced through the unfavorable position of very high building masses. Occasionally such effects can make presence in these open spaces nearly intolerable. Under some circumstances additional air hygienic burdens from dust whirls are also to be expected.
Map 04.03.2 Near Ground Wind Speeds at Night
In contrast to the day situation, the differences at night among the individual urban structures are in comparison to the reference station less noticable. The reason for this adjustment is the general decrease in the wind speed at night. So the wind speed itself for the reference station Tempelhof, calculated at a height of 2.70 m, has been reduced around 35 % and thus corresponds to the reduction in the long-term mean daily course for 1974 – 1990 (cf. Deutscher Wetterdienst 1993). Currently, opposite tendencies within the development structures can be observed. Above all areas with closed courtyards show no changes in the daily course of the wind speed. They are the worst ventilated in the night as well as the daytime. The wind speeds in areas with garden courtyards and in the forests are very slight both at night and during the day. The predominantly unsealed open spaces like farmland, green land, allotment gardens, but also the parks and similarly structured vacant areas exhibit the greatest reduction comparing day and night wind speeds. Here near ground cold air formation in built-up areas results in an additional stabilization of the upper air layer. Given the bad ventilation conditions with here, these areas are ranked as greatly at risk from emissions.
Moreover, it is striking that the high rise settlements are ranked two classes further down compared to the reference values. Here the large building masses mean that no stabilization of the near ground air layer can be expected. It can therefore be assumed that the night winds are too weak to produce the squalls which are noticable during the day. So the wind speed in the high-rise areas decreases compared to the day by around more than 60 %. High reductions compared to those at the freely exposed station Tempelhof as well as in the direct comparison with their day values are also displayed in the greened development areas. Beside the cooling down of vegetation areas, the daytime protective effect of tree structures contribute to this phenomenon.
The densely built-up, sealed areas of public facilities , utilities and special use exhibit minimal changes in either their relative values or their day values. Even the core areas exhibit similar qualities, the heat dissipation from the building mass prevents an effective stabilization the near ground air layer.